1. Field of the Invention
The present invention generally relates to implantable medical devices, and in particular, to the sealing of an implantable medical device for use within a human body.
2. Related Art
Implantable medical devices (IMDs), such as cochlear implant components, are commonly comprised of delicate mechanical and electrical components and upon implantation into a human body are typically subjected to long term exposure to relatively harsh environmental conditions due to, for example, the presence of conductive body fluids. As such, protection is often provided of the implanted device from the surrounding body environment to reduce the risk of failure of the implanted device and also to reduce the potential for leakage of any non biocompatible material from the implanted device into the body of the recipient.
Often, a combination of various sealing or protective methodologies is employed with IMDs. By way of example only, FIGS. 1 and 2 present an implantable receiver/stimulator 124 which corresponds to the implanted part of an exemplary cochlear implant system as known in the art. Receiver/stimulator 124 includes a receiver unit 132 and a stimulator unit 120. Receiver unit 132 is generally circularly shaped and incorporates a peripherally located internal antenna coil 133 having a centrally located attachment magnet 21 received within a pocket 22. Stimulator unit 120 incorporates the processing electronics which are non biocompatible for processing the signal received from receiver unit 132 and an extracochlear electrode (ECE) plate 129.
Extending from stimulator unit 120 is a first electrode lead 140 terminating in an electrode assembly 144 having a plurality of individual electrodes 142 and a second reference electrode lead 147 terminating in a reference ECE ball 148 comprising a rigid ball formed of platinum. Reference ECE ball 148 and ECE plate 129 both provide a return pathway for current applied at the electrodes 142 of electrode assembly 144 in the commonly used stimulation mode.
In order to provide an initial hermetic protective layer so that the various non biocompatible elements of the IMD are isolated from the environment upon implantation into a human body, the non biocompatible elements such as the processing electronics of the stimulator unit 120 are housed within an outer rigid shell or housing 121 formed of a biocompatible material such as platinum, titanium or palladium. Often, any component that is attached to an opening in housing 121 such as the ECE plate 129 is hermetically welded to the housing 121 in order to maintain the hermetic seal.
Another complication in hermetically sealing non biocompatible elements within an outer rigid shell or housing in an IMD such as the cochlear implant system depicted in FIGS. 1 and 2 is where a biocompatible component such as a wire formed of platinum is connected at one end to one or more of the non biocompatible elements of an IMD, such as electronic circuitry, and further to have a second end or region that is exposed to the body environment such as to convey or receive an electrical signal. Achieving this functionality requires a “feedthrough” region through which the component extends and which functions as a hermetic protective layer to separate the non biocompatible region of the IMD from the biocompatible region.
Again with reference to the illustrative example of the receiver/stimulator 124 depicted in FIGS. 1 and 2, there is shown in FIG. 3 the feedthrough region 300 which is located under the cap 11 (which is not hermetically sealed to housing 121) located on the underside of the stimulator unit 120 as illustrated in FIG. 2. Each of the 22 electrode wires 330 which form together electrode lead 140 and which terminate to form individual electrodes 142 of electrode assembly 144 are connected to individual platinum electrical contacts or pins 320 which at their other end are connected to the processor electronics contained within the housing 121 of stimulator unit 120.
In order to hermetically seal electrical contacts 320, feedthrough region 300 is filled with ceramic material 310 by a powder injection moulding or sintering process which functions to form a hermetically sealing protective layer or barrier, thereby preventing the ingress of body fluids to the non biocompatible regions of stimulator unit 120 or leakage of non biocompatible material to the body environment.
Another secondary level of sealing or a protective layer may be provided for an IMD by encasing or encapsulating the IMD within a flexible biocompatible layer. Encapsulating the IMD within such a protective layer can also provide other benefits such as improved handling due to the increased conformability and lubricity of the resultant encapsulated IMD.
In some instances, the outer surface of the encapsulating material may provide a surface finish less susceptible to the formation of a biofilm. Referring once again to FIGS. 1 and 2, in the case of a cochlear implant system, the receiver/stimulator body 124, electrode lead 140 and the reference electrode lead 147 are encapsulated within a protective layer of polymer such as polydimethylsiloxane (PDMS) or other type of silicone rubber to form a silicone shell 180 leaving electrically active regions such as the electrodes 142 of the electrode assembly 144, the reference ECE ball 148 and the ECE plate 129 exposed. These electrically active regions are typically formed of titanium given this material's combination of excellent electrical properties and biocompatibility.
Referring now to FIG. 4, there is shown a figurative sectional view of the flexible biocompatible protective layer 230 that would typically be applied to a cochlear implant system. A first layer of silicone adhesive 220 is applied to a substrate region 200 (corresponding to a surface of the IMD, for example), to form a covered substrate region 210 to facilitate the attachment of the protective layer 230 of silicone rubber, as generally silicone rubbers will not stick to metal surfaces without an adhesive layer. Silicone adhesive 220 is typically applied manually using a hand gun, although other automated processes may be used. Silicone rubber protective layer 230, which as referred to previously may be a PDMS material or other types of silicone rubber, is typically applied by an injection moulding process to form silicone shell 180. Other materials such as an insulating parylene coating or an initial primer layer may also be used depending on the nature of the substrate region 200. As depicted in FIG. 4, where there is an exposed or uncovered substrate region 215, a boundary region 250 exists between the covered substrate region 210 and any exposed substrate region 215.
Silicone shell 180 typically does not form a hermetic seal, as the silicone rubber material has some liquid permeability, but rather functions as a secondary protective layer to the hermetic sealing provided by the housing 121 and the feedthrough region 300. Still, this secondary protective layer aids in preventing the ingress of body fluids into the IMD or the leakage of non biocompatible materials from the IMD.
While generally these sealing methodologies are very effective, they can be improved. In the case of a cochlear implant, the seal durability is especially important due to the extended time that the implant is expected to remain in the body of the recipient (i.e., for periods of greater than 75 years). Seal durability is also important when the substrate or surface to which the protective layer is applied to includes an electrically active surface portion of the IMD (i.e., a surface portion that during operation of the IMD will conduct current often in a rapidly varying manner).